Sleep may look like inactivity, but the brain and body remain busy. NIH describes sleep as a complex biological process involving repeated cycles of non-REM and REM sleep, with changes in brain activity, breathing, heart rate and muscle activity across the night.
During deeper non-REM sleep, heart rate and blood pressure generally fall and the body enters a more restful state. REM sleep is associated with brain activity that can resemble wakefulness in some respects, vivid dreaming and temporary suppression of most large-muscle movement.
Two major systems help regulate when we sleep. Sleep pressure builds the longer we remain awake, while the circadian clock helps align sleep and alertness with an approximately 24-hour day. Light and darkness are important signals to this internal timing system.
Sleep also supports learning and memory. Different stages contribute to processing and stabilizing information acquired during the day. That helps explain why sleep loss can affect concentration, reaction time, decision-making and the ability to learn.
The cardiovascular system changes during sleep as well. NHLBI notes that heart rate and blood pressure usually fall during non-REM sleep. Chronic sleep deficiency has been associated with higher risks of several long-term health problems, while poor-quality sleep can also affect metabolism and immune function.
Sleep needs vary with age and individual circumstances, and duration is only part of the picture. Quality, regularity and timing matter too. A person may spend many hours in bed but still receive fragmented or insufficient restorative sleep.
The key point is that sleep is not simply unused time. While you are unconscious, the brain is organizing information and the body is regulating multiple systems needed for healthy function the following day.
Source: NIH/NHLBI — How Sleep Works and Why Is Sleep Important; NICHD — About Sleep and What Makes Us Sleep? General educational information, not individual medical advice.